Coated metal article



1943- M. e. WHITFIELD ETAL 2,455,457

COATED METAL ARTICLE 2 Sheets-Sheet l Original Filed 001;. 24, 1941 6 1 r t n 0 CI 5 1 n" m 1 H H u 1 m x e g .1 O

Dec. 7, 1948- M. G. WHITFIELD EI'AL COATED METAL ARTICLE 2 Sheets-Sheet 2 Original Filed Oct. 24, 1941 ATTORNE Y5 I I I I I I l I I I I I I I I .I

Patented Dec. 7, 194 a COATED METAL ARTICLE Maryland Original application-October 24,

Marsh'all G. Whitfield and Victor Sheshunofl,

Garden City, N. Y., assignors, 'ments, to Fairchild Engine poration, New York, N

by mesne assignand Airplane Cor- Y., a corporation of 1941, Serial No.

416,328, now Patent No. 2,396,730, dated March 19, 1946. Divided and this application July 24, 1945, Serial No. 606,855

3 Claims. 1

This invention relates to the metallic articles coated with other metals, and has particular reference to ferrous metal articles coated with aluminum or its alloys. p

This is a disivion of our application Serial No. 416,328, filed October 24, 1941, and issued March 19, 1946, as Patent No. 2,396,730.

Heretofore various methods have been proposed for coating ferrous metals with aluminum with a view toprocuring a composite structure which would withstand mechanical deformation, wide variation in temperature, and the like, Without separation at the zone between the two metals. These methods involved dipping the ferrous metal in molten aluminum, casting the aluminum on the ferrous metal, or spraying the ferrous metal with molten aluminum with or without the use of fiuxing or bonding materials. Few of these methods have been sufficiently satisfactory to justify their commercial use for all purposes. Also, these prior methods were complicated and expensive and the results of their use were so variable that production of uniform products was not at all certain.

In accordance with the present invention, articles are provided by applying a coating of aluminum, or aluminum base alloys, and other nonferrous metals having similar characteristics, to a dissimilar metal, such as one of the iron or steel metals, to any'desired degree of thickness while at the same time producing a bond between these metals which will withstand deformation, wide temperature changes, and other rigorous treatment.

Also in accordance with the present invention, particularly when applied to the coating of steel, the steel articles may be heat-treated to temper and harden the steel without the use of any other steps than those normally involved in the coating operation.

The preferred process for preparing articles embodying the invention is conducted essentially in four steps, beginning with a thorough cleaning of the base metal of all foreign matter, as by brushing or sand-blasting, to remove scale, dirt, and other material which would interfere with the formation of a strong bond. A chemically clean surface is not required; it is necessary merely to clean the surface sufficiently to remove scale, dirt and grease therefrom.

If only part of the ferrous article is to be coated, the portions not to be coated are covered with a suitable stop-off preparation which will prevent contact between those portions and the aluminum, or aluminum base alloy, and the ar- 2 ticle is then immersed in a bath of molten aluminum, or aluminum base alloys, for a suflicient time and at a proper temperature'to cause the aluminum to alloy with the ferrous metal and form a thin iron-aluminum alloy bond on the surface of the article. Preferably this coating operation is of short duration in order to preclude deep alloying action on the surface of the article, inasmuch as the iron-aluminum alloy formed thereupon is a hard, brittle metal which is not capable of withstanding considerable shock or distortion, and hence should not be so thick as to weaken the eventual bond.

A third step of the process consists of immediately transferring the article coated with the thin film of molten aluminum, to a second bath for applying a second film to the article. This second bath preferably has a lower melting point than aluminum or aluminum alloy of the first bath, and is maintained at a temperature below the critical temperature of the article, thereby also serving as a quenching medium for hardening the metal, but not so low as to cause the initial aluminum coating to freeze to any substantial degree. An alloy of aluminum having a melting point below that of aluminum has proven effective for that purpose. The article is retained in the quenching bath until its temperature has attained substantially the temperature of the quenching bath, and then a layer of aluminum of any desirable thickness is applied over the wetted surface remaining after removal from the quenching bath. This aluminum layer may be any desired thickness, and is preferably applied by casting it on the article, preferably by placing a mold about the wetted surface of the article and pouring molten aluminum in contact therewith to the desired depth or thickness. When the aluminum solidifies, a substantially homogeneous layer of aluminum is providedon the surface of the article which is secured by the bond of ferro-aluminum alloy to the ferrous article. This layer of aluminum may then be machined or otherwise treated to produce the finished or semi-finished product desired.

As an example of one form of article embodying this invention, an internal combustion engine cylinder, including barrel and head as a unit, or barrel and head separately, may be provided on its outer surface with a mufl of aluminum in the manner described and the muff provided with grooves or slots forming cooling fins, these grooves or slots either being formed by casting when the aluminum is applied to the cylinder, or by machining the aluminum muff so formed, as by saw-cutting or milling grooves in the metal. Cylinder barrels formed of appropriate types of steel and treated in accordance with the process outlined above are characterized by a high Rockwell hardness, toughness, and by a firmly seated and chemically-bound coolin structure aflixed thereto.

It will be seen that the articles embodying the present invention have added properties or facilities, such as, for example, the rapid cooling afforded by the aluminum because of its higher heat conductivity.

For better understanding of the invention, reference may be had to the accompanying drawing, in which:

Figure 1 is a view in vertical section illustrating the manner in which a cylinder barrel may be coated by dipping the same in a bath of molten aluminum;

Fig. 2 is a view in vertical section of a typical form of mold for casting aluminum about the cylinder barrel;

Fig. 3 is a plan view of the mold of Fig. 2;

Fig. 4 is a view in vertical section of a cylinder barrel with the aluminum muff applied thereto;

Fig. 5 is a view in vertical section of the cylinder barrel with the mud treated to provide cooling fins; and,

Fig. 6 are diagrammatic showings of typical heat treating operations in accordance with the present invention.

As an example of a method of producing articles embodying the invention, a cylinder barrel ID for an airplane engine is treated to produce a finished construction having cooling fins Ii thereon, as shown particularly in Fig. 5 of the drawings. The particular cylinder barrel In, shown by way of example, may consist of a tubular member of any desired dimensions having a radially directed flange l2 thereon and a suitably raised annular ring i3. No special surface finish, such as roughening, need be provided to aid in securing the adhesion or binding of the aluminum thereto.

Various types of steel may be used in the preparation of such cylinder barrels, typical of which is high strength SAE 4140 steel (a chrome-molybdenum steel, containing (LS-1.1% chromium, 0.15-0.25% molydenum, 0.35-0.45% carbon, 0.fi-0.90% manganese, and not more than 0.04- 0.05% phosphorous and silicon each).

Inasmuch as it is desired to coat only a portion of the exterior of the barrel l0, namely, that portion above the flange II, the remainder of the barrel, including the inside and the flanges, may be coated with a stop-oil preparation, such as, for example, a lime solution or equivalent coating. The barrel is then supported on a ring ll which may be formed of steel, iron, or other material capable of withstanding the high temperature of the coating bath and which is provided-with a handle l by means of which the cylinder It may be lowered into bath IS. The bath Ii which may be an externally heated crucible Ilia or the like, is partially filled with molten aluminum, or an aluminum base alloy, and maintained at a temperature between about 1525 and 1625 F. The temperature of the bath I5 depends upon the melting point of the aluminumcontaining metal of which it is composed, but in any case its temperature is maintained somewhat above its melting point, as indicated.

An annular collar or sleeve i1 is supported loosely on the upper surface of the flange 12 of the cylinder barrel and is lowered into the molten aluminum with the cylinder barrel Ill. The collar I1 is provided with outwardly projecting arms I! which engage the top edge of the crucible lia so that as the cylinder barrel I0 is lowered into the bath the collar II will be stopped by engagement of arms IS with the top edge of the crucible l6a. The descending barrel ill thus drops below the collar l1 and allows the molten aluminum to flow between its lower edge and the flange I! on the cylinder barrel.

The function of the ring I4 and the collar I1 is to part and push aside and thus prevent contact of the slag or aluminum oxide on the top of the bath with the portions of the cylinder barrel id to be coated. Thus, as the cylinder barrel iii and the ring ii are lowered into the bath IS the aluminum oxide will be parted and will flow around the cylinder barrel and upwardly around the collar l1. When the collar I1 and the flange 12 separate, the molten metal beneath the oxide scum flows around the upper portion of the cylinder barrel and comes into intimate contact with it.

Temperatures between about 1525 and 1625 F. are used to promote a very quick formation of a ferro-aluminum alloy at the engaging surfaces of the bath l5 and the cylinder barrel III. A suflicient bond between the aluminum and the cylinder barrel in will be obtained within one to six minutes, or in the case of small articles, such as, for example, wire, within a few seconds. Inasmuch as only a thin layer of the ferro-aluminum allow is desirable in most instances, the formation of the alloy is arrested by cooling the coated barrel III. This may be accomplished by immersing it in a second bath of molten aluminum or an alumnium base alloy, at about the melting point of aluminum or slightly below, preferably between 1000* and 1250 F. Preferably a bath containing aluminum and silicon is used, with the silicon depressing the melting point of the aluminum. For example, a suitable bath may contain about 13% silicon. The use of such an alloy bath also has the added advantage of cleaning the surface of the aluminum coating on the cylinder barrel. It appears that when the highly heated barrel is immersed in the lower temperature bath, the heat of the barrel raises the temperature of the bath adjacent the barrel surface and the consequent increased fluidity of the bath washes or strips the oxide from the surface of the aluminum coating. The second bath thus arrests the alloying action and also places the surface of the barrel in condition for the subsequent casting operation. The reduced temperature of the barrel also facilitates the subsequent casting operation because the temperature is more nearly equalled.

During the initial coating operation in bath IS, the cylinder barrel I0 is heated to a high temperature, such that solution of the constituents of the steel takes place, thus placing the entire barrel in a condition to be hardened by subsequent quenching.

Inasmuch as it is known that many steels, when allowed to cool in air or quenched in oil from temperatures on the order of 1525 F. will form an extremely hard but brittle martensitic structure in the metal, advantage is taken of the use of. the second bath to quench the steel rapidly to a temperature below the critical, i. e., below the beginning of precipitation of ferrite from the austenite on cooling. B regulating the subsequent treatment of the steel, the desired properties are obtained. This quenching operation resuits from the immersion of the barrel in the second bath of aluminum or an aluminum base alloy maintained at a temperature below the critical temperature of the steel involved, but preferably above martensite formation temperature.

A bath of this type has several exceedingly valuable advantages, namely, 1) a rapid quench which results from theespecially good heat con ductivity of the molten aluminum or aluminum base alloy, of which the second bath is composed, (2) the great heat extraction capacity of the aluminum and aluminum base alloys enables the close control of the quenching temperature.

It will be observed that both in the first bath and in the second bath the steel is perfectly wetted by the aluminum or aluminum base alloy of the bath, so that there is virtually a continuous heat transfer path between the interior of the steel member and the highly conductive bath. whereby travel of heat in and out is facilitated. This method thus contrasts with lead and other molten metal bath quenching methods where the article to be heat-treated is not wetted and hence heat transmission is imperfect at the interface.

Inasmuch as the steel has been perfectly wetted with aluminum in the first coating bath, immersion in the second bath at a lower temperature arrests the otherwise rapid formation of the.

I ferro-aluminum alloy bond, and in addition hardens the cylinder. Cylinders treated in accordance with our process have, inmany instances, a hardness of 55-58 Rockwell C.

When the article has been quenched in the second bath, and the formation of the ferro-aluminum bond arrested, the cylinder barrel, while aluminum, is transferred to a mold where molten aluminum at its melting point, or slightly above, is poured about the wet cylinder barrel and cooled to form the aluminum muff 30 of any required thickness on the ferroaluminum alloy film 29 extending over the selected portion of the exterior surface of the barrel, as shown in Fig. 4. Thequenching in the second bath purposely reduces the temperature of the article below that of the casting metal to preclude reheating of the article during casting to a temperature where the steel structure will be changed. The steel is accordingly not reheated to its critical temperature. Where heat treatment of the steel is not necessary or desirable, the second or quenching bath may be omitted, and the muff 30 out immediately on the aluminium skin overlying the thin ferro-aluminum alloy film 29 formed on the article In in the first bath and while said skin is still wet as described.

Figs. 2 and 3 of the drawings disclose a suitable type of mold M for casting the aluminum mufl 317 on the cylinder barrel ID. This mold may be of the split type, including two segments and 2|, in the form of halves of a hollow cylinder having overlapping hinge flanges 22 and thereof, which are joined by hinge pins 60 permit the two sections to be spread apart. Adjacent the lower end of the mold M in each section 20 and 2| is an inwardly projecting shoulder 24 forming an annular support for the flange [2 on the cylinder barrel. A tapering opening 25 is also rovided for receiving the lower end of the'cylinder barrel Ill.

The cylinder barrel l0, still wetted by the molten aluminum, or aluminum base alloy, is placed in the mold 26 interposed between the flange l2 thereon and the shoulder 24 in the interior of the mold. Molten aluminum can then be poured into the M with an asbestos washerupper end of the mold between its wall and the outer wall of the wet cylinderbarrel ill to produce a muff which is bonded securely to the cylinder barrel ill by the alloy bond described above.

The casting may be cooled in air and cooling may be hastened by blowing air through the interior of the cylinder barrel I 0. When the aluminum is hardened, the mold may be split to permit removal of the cylinder barrel. The casting may then be quenched in oil.

Thereafter the aluminum mun 30 on the barrel may be machined in any suitable way to cut slots therein and to remove unwanted portions thereof to form cooling fins II which are bonded tightly to the cylinder barrel, and thus are capable of quick and efficient transmission of heat for cooling purposes from the interior of the cylinder l0 during its cooperation with the piston P in operation of the engine.

If desired, prior to machining of the muif 30 on the cylinder barrel, the article may be transferred to a holding furnace for tempering purposes and thereafter quenched in oil.

It will be understood that the temperatures of the various baths and the times of treatment of the articles being coated and hardened will vary considerably, depending upon the size of the article, and hence its rate of heat transfer, and the type of metal undergoing treatment. However, it has been found that the most satisfactory temperatures for most types of steel in the initial alloying and coating bath will lie in the range between 1525 and 1625 F. The time of immersion in this bath varies widely. For example, in coating wire it has been found that a few seconds are oftentimes sufficient to successfully form a strong ferro-aluminum bond, whereas larger articles may require as much as ten minutes or even more to heat the surfaceto be coated to the alloying temperature necessary to .form a satisfactory bond.

The quenching temperature also may be varied, depending upon the type of micro-structure to be formed in the metal, but we have found that a temperature between 1000 most instances, satisfactory steels.

cylinder barrel of the type described above, a quenching period of one-quarter to five minutes has been found to be entirely satisfactory.

As illustrated diagrammatically in Fig. 6, the sequence-0f steps of the article, for example, a piece of steel, is raised in temperature to between about 1575 and 1625 F., then quenched at about 1100 F. in an aluminumsilicon alloy bath. The quenched article is then quenched in oil, drawn at about 1000 F., and quenched again in oil. Optionally, depending upon the hardness and other physical properties required, the article may be placed in a draw furnace, after the first quench in the second bath, for a suitable period, followed by an oil quench. This is indicated by the dotted curve of Fig. 6.

The high heat conductivity of aluminum at the temperatures involved (1.01 c. g. 5. units at about 1110 F.) and the high specific heat of aluminum (.2683 calorie per gram at 1110 F.) provides a particularly rapid quenching and heat-dissipating action and thus allows accurate control of the heat-treating operation.

From the foregoing disclosure of a typical article embodying the present invention, it will be clear that we have provided coated articles to which the coating metal is bonded with great tenacity so that a substantially unitary heat conductive path is provided from the ferrous metal through the coating, and also articles having a high hardness with adequate toughness to withstand the rigorous treatment aflforded such articles as cylinder barrels for aeronautical engines are provided, notwithstanding the high temperatures and widely different coeflicients of thermal expansion of the aluminum and the steel.

It will be understood from the foregoing that the process disclosed herein is susceptible to considerable variation in the temperatures and times of treatment, depending upon the characteristics of the article being treated, its composition, and the physical characteristics desired in the finished products. Therefore, the process and the article embodying the invention disclosed herein should be considered as illustrative only and not as limiting the scope of the following claims.

We claim:

1. An article of manufacture comprising an annular steel cylinder, a relatively thick annular casting of an aluminum-containing metal on an.

exterior portion of said cylinder, and a ferroaluminum alloy film at the interface between said cylinder and the casting forming a bond uniting the two metals permanently, whereby a continuous heat-conducting path is formed through the steel cylinder, the term-aluminum bond and the aluminum-containing casting, said casting having a thickness materially in excess of said ferroaluminum alloy film.

2. A cylinder adapted tolcooperate with a piston therein, comprising an annular steel cylinder, a relatively thick casting of aluminum-containing metal of the class consisting of aluminum and aluminum base alloys on the exterior surface of said cylinder, a term-aluminum alloy film at the interface between said cylinder and the casting forming a bond unitin the two metals permanently, whereby a continuous heat-conducting path is formed through said cylinder wall, the ferro-aluminum alloy and the casting, said casting having a thickness materially in excess of said farm-aluminum alloy film, and heat-radiating fins extending from the surface of said casting.

3. An article of manufacture comprising a ferrous metal article, a relatively thick casting of an aluminum-containing metal on at least one surface portion of said article, and a ferroaluminum film at the interface between said article and the casting forming a bond uniting the two metals permanently, whereby a continuous heat conducting path is formed through said article, the ferro-aluminum bond and the aluminum-containing casting, said casting having a thickness materially in excess of said ferroaluminum alloy film.

MARSHALL G. WHITFIELD. VICTOR SHESHUNOFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,311,242 Norton July 29, 1919 1,528,947 Preston Mar. 10, 1925 1,605,601 Mougey Nov. 2, 1926 1,710,136 Angle et a1 Apr. 23, 1929 1346,49; Norton Feb. 11, 1930 2,012,739 Brown Aug. 27, 1935 2,082,622 Fink June 1, 1937 2,303,869 Quinlan et al. Dec. 1, 1942 

